Impossible to know what his value will be but the highest is on the last day of the season, the lowest on the first day of the season. I would imagine it will go up by around 15% with his next skill point.

It is debatable at what time of the season is the best to sell because as the season progresses and the player's value increases there is less interest in him from the managers (other than maybe level 1 as the new players/teams come in the game). In the beginning of the season when his value is the lowest there could be a bidding war for him and his sales price could exceed his value from the end of the prior season by quite a bit.